It is often necessary to separate a light beam into its orthogonally polarized components. These components are usually termed "P-polarized" ("P" for parallel) and "S-polarized" ("S" for "senkrecht") with regard to tilted interfaces between two isotropic media, or, alternatively, "O-polarized" ("O" for "ordinary") and "E-polarized" ("E" for "extraordinary") with regard to uniaxial optical materials (typically crystals). Devices which separate light into components of differing polarization are termed "polarization separators", "polarizers" or "beam splitters". Separators can be used as combiners, to combine separately polarized beams into a beam of mixed polarization (i.e. a beam having components of varying polarization).
For the purposes of this disclosure, the following definitions should be noted:
A walk-off plate is a slab of birefringent material with entering and exit surfaces mutually parallel. The optic axis is not parallel nor perpendicular to these surfaces. Light entering is separated into two paths, depending on polarization. On exit, the two possible paths are re-refracted parallel to the initial, entering direction, hence mutually parallel, but displaced from one another. PA1 A walk-off device is any means to separate polarizations into parallel, displaced beams. Examples of walk-off devices are a single, or several cascaded, walk-off plates, and also the various embodiments of the present invention.
FIGS. 7 through 11 show a number of the various prior-art devices for polarization separation: a walk-off plate (FIG. 7), polarization cube (FIG. 8), Glan prism (FIG. 9) and Nicol prism (FIG. 10). There are other prism designs, not shown, such as Wollaston, Rochon or Senarmont prisms, which differ among the group in the direction and amount of beam deflection, but which are similar in concept to the tilted interface between uniaxial media of FIGS. 9 and 10.
FIG. 11 shows a polarization cube using a polarizing interface (polarization beam coating), as taught by Asanuma, Japanese patent application no. 2-168204(A), FIG. 4 (published Jun. 28, 1990). A glass prism (130) has a glass plate (132) attached, with the polarization beam coating (131) between. The incoming beam (134) is split at the polarization beam coating (131), with one component of light (135) being reflected by the interface, and the other (136) passing through the interface (131) into the plate (132). The beam is reflected off the rear surface (133) of the plate, and then passes back through the prism (130) and exits parallel to the other beam (135). For clarity, it should be noted that in the drawings, the dashed line represents a beam of a particular polarization, the dotted line represents the orthogonally polarized beam, and the alternately dashed-dotted line represents a beam of mixed polarization (viz. unpolarized, polarized but not of said particular polarization nor orthogonal to it, or partially polarized).
As shown in FIGS. 8 through 10, most of the prior-art polarization separators resulted in at least one of the polarized light paths being sent off at an angle from the incoming light, the two polarized beams are not mutually parallel.
Uniaxial materials are made of specific substances which are characterized by having a unique axis of optical symmetry, called the optical axis, which imposes constraints upon the propagation of light beams within the crystal. Two polarization modes are permitted, either as an ordinary ("O-polarized") beam polarized normal to the optic axis, or as an extraordinary ("E-polarized") beam polarized in a plane containing the optic axis and the direction of propagation. Each of the beams' polarization has a different associated refractive index. It is this polarization dependent refractive index that enables suitably cut and oriented prisms of birefringent materials to act as polarizers and polarizing beam splitters. Birefringent materials commonly used include calcite, rutile, crystal quartz, ammonium dihydrogen phosphate and magnesium fluoride.
If it is required that the separated light paths should be parallel, then the prior art device of choice was the "walk-off" plate made of linear birefringent material shown in FIG. 7. In a walk-off device (71), the incoming beam (72) is split into two orthogonally polarized beams by the birefringent nature of the crystal. The angle of one of the refracted beams obeys Snell's law; that beam is termed "O-polarized". The E-polarized beam (73) is refracted away from the path of the incoming beam (72), and yet exits parallel to the O-polarized beam (74) which is refracted according to Snell's law.
Each of the various birefringent materials have associated problems, including brittleness, softness, expense, hygroscopic effects, and so on. It is desirable, therefore, to find a substitute for the walk-off plate and other polarization separators which achieves the result without the associated problems. It is also desirable to work with parallel, orthogonally polarized beams, such as with the walk-off plate. In this specification, such novel devices with parallel beams will be termed "walk-off devices", as a generalization of the "walk-off plate".
Referring to FIG. 2, a slab of isotropic, transparent material (10) is shown, with its surface (11) having known characteristics or a specific coating. Two beams of light (14) and (15) of orthogonal polarization are shown striking the surface coating (11) at an oblique angle .theta. to the perpendicular (12). Here, the "P-polarized" beam (14) is shown dashed, and the "S-polarized" (15) beam is dotted. As each beam strikes the surface, some of each beam is transmitted, and some is reflected. The transmitted portion is shown as "T", and the reflected portion as "R", with the subscript showing the polarization of the beam ("T.sub.s "). T and R are "fractional powers"--that is, they take on values between 0% and 100%. The transmitted beams T.sub.s and T.sub.p are refracted at an angle .phi., as they enter the surface of the material. The relationship of .theta. and .phi. is given by Snell's Law: EQU n.sub.air sin .theta.=n.sub.material sin .phi.
For purposes of illustration, air is assumed for the incident medium; more generally, other incident media can be considered.
If there is no absorption at the coating or surface, then by power conservation, T+R=100%, specifically T.sub.p +R.sub.p =100% and T.sub.s +R.sub.s =100%.